The latest terrorism scare, involving a pair of explosive packages bound for Chicago from Yemen, has shed light on a new target for bombers—aircraft traveling to the U.S. whose cargo holds either have not been inspected, or if they have, by x-rays and bomb-sniffing dogs that are not sensitive enough to root out certain types of explosives. Would-be aircraft bombers have proved successful at smuggling pentaerythritol tetranitrate (PETN) into aircraft cabins concealed in shoes and underwear, but fortunately have been unable to detonate this high-powered, military-grade explosive in flight. The latest plot placed one PETN bomb and detonator in a cargo plane and another in the cargo hold of a passenger plane, where they were less likely to be detected.

One of the bombs, which contained 400 grams of PETN, was found at the East Midlands Airport in England, having traveled there on board a United Parcel Service cargo plane. The other device, which contained 300 grams of PETN inside a Hewlett–Packard desktop printer, was found in a FedEx package in Dubai after having traveled there on a Qatar Airways passenger flight. Both bombs contained far more explosive material than the 80 grams of PETN that Umar Farouk Abdulmutallab managed to smuggle onto a Northwest Airlines flight in his underwear while en route from Amsterdam to Detroit on Christmas day last year. Abdulmutallab's plan failed when he was unable to detonate the explosives. In December 2001 Richard Reid tried to use PETN hidden in his shoe to blow up an American Airlines flight from Paris to Miami.

The TSA expanded its use of Explosives Trace Detection (ETD) technology at security checkpoints around the country to screen carry-on baggage and passengers for traces of explosives after the failed Christmas Day attack. When using an ETD device, officers swab a piece of luggage or passenger hands and then place the sample inside the ETD unit, which analyzes the content for the presence of potential explosive residue. The U.S. Department of Homeland Security has awarded $15 million in American Recovery and Reinvestment Act funding for 400 fixed ETD units. The federal government's fiscal year 2011 budget includes $39 million to purchase about 800 portable ETD machines. (There are about 150 international airports in the U.S.)

The October 29 incident indicates that more attention must be paid to screening shipped packages. Yet, even if all cargo were screened, explosives, including PETN are difficult to detect because they have a very low vapor pressure, which means very little of the explosive material gets into the air around the bomb where it can be detected. "Normally, whenever there is a solid material sitting on a surface, there is a certain concentration of that substance lingering in the air above it in a gas phase," says physicist Kurt Becker, Polytechnic Institute of New York University's associate provost for research and technology initiatives. "All explosives are notorious for having a very low vapor pressure at room temperature." Cheese, by comparison, has a very high vapor pressure at room temperature and is easy to detect through its aroma.

"You need to look for some type of detonator, which is easier to find than the explosives themselves because most types of detonators have metal in them—a wire or a microchip, for example—that triggers a small spark or electrical signal," Becker says.

Becker serves as a consultant for Austria-based gas analysis instrument-maker Ionicon Analytik, GmbH, which makes proton transfer reaction–mass spectrometry (PTR–MS) technology that the company claims can distinguish substances having very similar molecular structures as well as correctly identify explosives, chemical warfare agents and substances that could be combined to create a bomb. Ionicon's goal is to have its technology deployed throughout the airport screening process, both for passengers and packages. "We have a proposal pending for the Department of Homeland Security that would further develop the technology," he adds.